The present invention relates to a pneumatic machine tool particularly suitable for use in the aeronautical industry.
Pneumatic machine tools are in very widespread use and are often preferred to electrically driven machine tools because of their performance.
One type of prior art pneumatic machine tool includes a driving shaft driven in rotation by pneumatic drive means, a driven tool-carrier shaft, a gear mechanism disposed between the driving shaft and the driven shaft to drive the latter, a set of sensors for measuring operating parameters of the machine tool, and a central control unit in which are stored algorithms for controlling machining as a function of data received from the sensors, the gear mechanism including means for changing the gear ratio under the control of the central unit.
The operation of this type of machine tool can be adapted to start predetermined operating cycles such as the lubrication of a shaft carrying the tool, safety cycles, cycles for advancing and withdrawing the tool as a function of the depth of the bore when the machine tool is used as a drilling machine, etc., as a function of operating parameters of the machine tool supplied by the sensors, for example the position of the tool relative to the workpiece to be machined, the type of tool, etc.
The object of the invention is to provide a machine tool of the aforementioned type which is additionally capable of starting preprogrammed operating cycles at two different speeds as a function of machining parameters.
It therefore consists in a pneumatic machine tool of the aforementioned type characterized in that the gear mechanism includes primary and secondary sets of toothed wheels with different demultiplication ratios, each adapted to rotate freely relative to the driving shaft, the speed changing means including a positive clutch constrained to rotate with the driving shaft and axially mobile relative thereto under the control of a pneumatic control unit controlled by the central unit, between two end positions in which the positive clutch is engaged with respective sets of toothed wheels.
The machine tool in accordance with the invention can also have one or more of the following features, individually or in all technically feasible combinations:
the secondary set of toothed wheels is part of the primary set of toothed wheels;
the positive clutch has radial teeth on opposite end faces of the positive clutch adapted to cooperate with complementary radial teeth carried by one of the toothed wheels of each set of toothed wheels;
the pneumatic control unit includes a piston fastened to the positive clutch and mobile in a chamber and means for supplying the chamber with pressurized fluid on either side of the piston, including a solenoid valve under the control of the central unit;
at least part of the gear mechanism is removable from the machine and constitutes an interchangeable modular assembly;
the central unit has stored in its memory an algorithm for monitoring the rotation speed of the driven shaft by comparing the actual rotation speed of the driven shaft and maximum and minimum permissible rotation speeds between which lies the rotation speed imposed by the central unit;
the primary set of toothed wheels is a speed multiplier or reducer.